gitignore: add .idea folder to .gitignore
Checklist
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commit message follows commit guidelines
Affected core subsystem(s)
tools
Description of change
Add .idea/ to .gitignore to make it easier to work on Node with JetBrains IDEs.
Covers all JetBrains IDEs: IntelliJ, AppCode, PyCharm, CLion, Android Studio, Webstorm and others
Similar to recently landed PR #7967